Refrigeration systems are prevalent in our everyday life. Refrigeration systems can be found in such varied locations as automobiles, commercial and residential refrigerators and freezers, commercial and residential air conditioning systems, supermarket display cases and many other applications.
Vapor compression refrigeration systems typically comprise four elements: a compressor, a condenser, an expansion device and an evaporator. Refrigeration systems operate on the basis of a heat or thermodynamic cycle. In conventional refrigeration systems, a refrigerant is utilized which has a lower boiling point than the space which is to be cooled, i.e., have heat removed therefrom. In the evaporator, heat is passed through the evaporator coils to the liquid refrigerant which absorbs that heat as the heat of vaporization. The phase change of the refrigerant from a liquid to a gas which occurs in the evaporator carries the absorbed heat with it. The gaseous refrigerant is withdrawn from the evaporator by a compressor which then compresses the gaseous refrigerant. The compressed vapor is discharged from the compressor to the condenser. In the condenser, the refrigerant once again undergoes a phase change whereby the heat of vaporization is released to the condenser's surrounding. The refrigerant then condenses and changes from a gas to a liquid. The liquid refrigerant then is passed through an expansion device to the evaporator where the heat cycle begins again.
In order to lubricate the moving parts of a refrigeration system compressor, the refrigerant usually includes a lubricant. In chlorofluorocarbon refrigerants ("CFCs"), the conventional lubricant is mineral oil. The mineral oil is miscible with the liquid CFCs.
The phase out of CFC-12, under the terms of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer is effecting an immediate shift away from CFCs in refrigeration systems toward hydrofluorocarbon refrigerants ("HFCs"), such as HFC-134a, a substitute refrigerant with no ozone depletion potential. However, one may not merely substitute HFC-134a for CFC-12. When CFC-12 is removed from a refrigeration system, residual mineral oil remains in the system. When HFC-134a is added to such a system, it is not miscible with the mineral oil used to provide lubrication in the CFC-12 system. Due to this immiscibility, synthetic lubricants, such as, polyolesters, have been developed specifically to mix with HFC refrigerants and provide proper lubrication. Problems result from this immiscibility. At temperatures occurring in the evaporator, very little HFC-134a is dissolved in the mineral oil, resulting in a high viscosity lubricant that does not circulate. Instead, the mineral oil trapped in the evaporator interferes with refrigerant flow and heat transfer. Processes developed for removing mineral oil during a retrofit, i.e., a change from one refrigerant to another, have been developed. The prevailing "triple flush" procedure safely and efficiently removes mineral oil from the system. The "triple flush" procedure comprises diluting the mineral oil in large amounts of polyolester lubricant. Repeated replacement of the compressor lubricant after periods of system operation result in progressively lower mineral oil levels. Although the "triple flush" procedure is effective, it has high costs associated with lubricant, waste disposal and technician labor.
Opinions differ widely on how much residual mineral oil is acceptable after retrofit. Recommendations from compressor manufacturers and refrigerant manufacturers range from 1% to 8%. While the differences between these values may not seem large, reducing the residual oil content from 8% to 1% may involve a significant effort and cost. Accordingly, there is a significant need for an apparatus and a method of removing immiscible lubricants, such as mineral oil, from refrigeration systems in both an effective and a relatively economical manner.